This invention relates to marine seismic prospecting and, more particularly, to a connection system for attaching equipment to and detaching equipment from marine seismic cables.
A marine seismic streamer is a cable, typically several thousand meters long, that contains arrays of hydrophones and associated electronic equipment along its length. One purpose of the streamer is to position the hydrophone array at a known depth and orientation relative to a towing vessel in a survey area. Externally mounted equipment, such as depth controllers, called xe2x80x9cbirds,xe2x80x9d emergency recovery pods, and acoustic pods, performs the functions of positioning and controlling the cable. Individual units of these kinds of external equipment are attached to the streamer at various positions along its length. All of these external units should be both attached to and removed from the cable as quickly and reliably as possible. Operational expenses of seismic vessels require rapid attachment and detachment of these external units. Because these external units typically cost thousands of dollars, they demand the highest degree of reliability from any attachment scheme. Cable attachment failures caused by connector failures or by cable accidents result in a significant financial loss both in time and in expensive equipment.
Today""s typical cable attachment solutions consist of a collar arrangement that relies on a hinge and latch mechanism for operation. Examples of these mechanisms are described in U.S. Pat. No. 5,507,243, xe2x80x9cConnector For Underwater Cables,xe2x80x9d Apr. 16, 1996,to Oneil J. Williams et al. and in U.S. Pat. No. 5,709,497, xe2x80x9cLatching Device,xe2x80x9d Jan. 20, 1998, to David W. Zoch et al. External units attached to the collars are clamped around races on the cable as the cable is payed out from the back deck of a survey vessel. The races allow the cable to rotate inside the collars while the external units do not rotate as they are towed along. Conventional connector schemes usually require one operator to position and hold the awkward external unit in place while a second operator secures the manual latching collars to the cable, often while trying to maintain balance on a rolling survey vessel. Requiring two operators significantly increases the cost of operation.
These conventional mechanisms also incorporate springs or pins having dissimilar metals in contact with the collar. Dissimilar metals in contact in seawater corrode because of galvanic reactions. While conventional hinge-and-latch collars offer quick attachment and removal when new, exposure to salt water degrades their performance and can eventually lead to their complete failure. A failed collar can result in the loss of an external electronic unit or a jammed connector on the seismic cable, which costs time in removing external devices as the cable is reeled in.
A thick cross section (typically of aluminum) is required to safely imbed a conventional latching mechanism within the collar. Such a large cross section creates hydrodynamic noise and lateral accelerations on the seismic cable as it is towed through the water. These undesirable characteristics corrupt the sensitive measurement of seismic acoustic signals by the hydrophones.
Clearly there is a need for an apparatus and a method for avoiding these serious shortcomings that significantly add to the cost of a seismic survey at sea.
This need is satisfied and other shortcomings are overcome by an innovative cable connection system having features of the invention. The connection system includes a cuff attached to a piece of equipment to be connected to the streamer cable at a known location. The C-shaped cylindrical cuff has a circular inner surface interrupted by a gap. A throat is formed by the gap in the cuff extending the length of the cuff. The spacing between the ends of the C across the throat defines the width of the gap. The width of the gap is slightly larger than the diameter of the streamer cable so that the cuff can slip onto the cable. An inner collar having a race is affixed to the cable at a known location. The diameter of the race is greater than the width of the gap formed by the cuff""s throat. The inner surface of the cuff can be slid into position on the race of the inner collar. Because the diameter of the race exceeds the width of the gap of the throat, the cuff and the attached equipment cannot disconnect radially from the inner collar. Structural elements on the collar further prevent longitudinal displacement of the cuff along the inner collar. In this way, equipment is connected to the cable at a known location.
In a preferred version of the system, the inner collar has a circumferential groove that admits a retractable pin extending from the external unit through an aperture in the cuff. The groove constrains the pin and prevents longitudinal movement of the external unit along the cable and off the inner collar. A single operator can remove the unit with the cuff from the inner collar by manually retracting the pin and sliding the cuff longitudinally off the inner collar and then radially off the cable. In an alternative version, the inner collar has a raised circumferential shoulder just aft of the race to act as a precise longitudinal stop for the cuff. Forward motion of the cable through the water will hold the cuff against the shoulder. Preferably, the cuff itself is of unitary construction with no moving parts.
Thus, the novel connection system includes a cuff having no moving parts and no dissimilar metals in contact to provide a connector that is significantly more reliable, even after long exposure to seawater. The connection system does not require two operators as do conventional systems. One operator suffices because the connection system requires no activation of a latch for a secure attachment.